A telecommunication network is shown schematically in FIG. 1. Mobile terminals 10 (also termed user equipment, or ‘UE’s) such as mobile phones, computers, PDAs, etc, are able to wirelessly transmit data to, and wirelessly receive data from, various base stations 20 (also termed ‘BS’s). Each base station may be in communication with a wired network 30, such as an optical network. The telecommunications network may be controlled by a network controller 40. The present invention relates primarily to the radio access portion of the network (i.e. to the wireless communications between the mobile terminals and the base stations), and so the remainder of the network will not be discussed in further detail.
Transmissions from a base station to a mobile terminal are generally termed ‘downlink’ transmissions, whilst transmissions from a mobile terminal to a base station are generally termed ‘uplink’ transmissions. Such transmissions may be either Frequency Division Duplex (FDD) or Time Division Duplex (TDD). In FDD, downlink and uplink transmissions are made in separate frequency bands, such that packets can be transmitted in the downlink and uplink directions at the same time. In TDD, on the other hand, downlink and uplink transmissions are made on the same frequency band and are transmitted in different predetermined time slots.
TDD is flexible in that the duration of downlink and uplink transmissions can be configured depending on the traffic intensity in the downlink and uplink directions, thus allowing for connections with asymmetric transmission schemes. For example, the time allocated to downlink traffic might be greater than the time allocated to uplink traffic for downlink intensive systems, and vice versa for uplink intensive systems. The present invention was conceived with LTE TDD in mind, which is generally downlink intensive.
In LTE, data is multiplexed in the downlink using orthogonal frequency division multiplexing (OFDM), whilst in the uplink single carrier frequency division multiple access (SC-FDMA, also known as discrete Fourier transform OFDM, or DFT-OFDM) is used.
Data is scheduled in radio frames with a periodicity of 5 ms or 10 ms. An example of a radio frame with 5 ms periodicity is shown in FIG. 2. Each frame 3 is 10 ms in duration, and comprises two sub-frames 5 of duration 5 ms. Each frame is split into transmission time intervals (TTIs). Some TTIs (those marked ↓) are scheduled for downlink transmissions, whilst others are scheduled for uplink transmissions (those marked ↑). It can be seen that there are more downlink TTIs than there are uplink TTIs (in this case the ratio of downlink time to uplink time is 3:1).
LTE requires a terminal that is receiving a transmission to transmit a feedback report to the sender of the transmission to confirm whether or not a scheduled transmission was received, and/or whether it was received correctly (various prior art methods exist for determining whether data is received correctly, and so that is not discussed herein). If the terminal receives the transmission correctly, then it is required to transmit an acknowledgement (ACK) to the sender. If it does not receive the transmission correctly (perhaps because it determines that errors have been introduced into the data, e.g. by interference during the transmission) then the terminal is required to transmit a negative acknowledgement (NACK) back to the sender, and the sender is required to retransmit the data. Such feedback communications take place on one or more channels dedicated for that purpose, which are often shared between multiple mobile terminals. The base station is able to determine which ACK/NACKs originate from which terminal, because each terminal is assigned a unique code with which it encodes its data before transmitting that data. As in code division multiplexing (CDM) the base station is able to distinguish between transmissions from various mobile terminals because the codes assigned to those various terminals are orthogonal (in the case of synchronous CDM) or pseudorandom (in the case of asynchronous CDM). A suitable type of code is the constant amplitude zero autocorrelation (CAZAC) code.
It can be seen that where the ratio of downlink:uplink is not 1:1 the requirement for feedback reports becomes problematic. Because more data is being sent in one direction than in another it is not simple to schedule one feedback report for each data packet that is transmitted.
Prior art methods have addressed this problem by bundling ACK/NACK data together. For example, FIG. 3 shows a situation where there are four downlink data streams for every one uplink. Three of the data streams (those marked ACK) are received successfully, whilst one is not (marked NACK). Because it is not possible to transmit four feedback (without increasing the payload of the upload signal, thus reducing the overall uplink performance), those feedback reports are combined using an ‘AND’ operation, where NACK takes precedence over ACK. The result is that a NACK feedback report is transmitted to the sender, meaning that all the data, including that which was received successfully, must be retransmitted.
Thus it can be seen that ACK/NACK bundling, whilst it can improve the performance of the uplink control channel (in LTE: the physical uplink control channel, or PUCCH), it can increase the burden on the downlink transmission (in LTE: the physical downlink shared channel, or PDSCH). That is, ACK/NACK bundling may lead to needless downlink re-transmission, which is inefficient as it reduces the downlink transmission throughput.
It is an object of the invention to alleviate some of the problems discussed above, by proposing a more efficient ACK/NACK bundling scheme. As will be discussed, however, the proposed solution has a wider application, and does not solely relate to an ACK/NACK bundling scheme.